Chien Chih Liu

Enhancement of the light output performance for GaN-based light-emitting diodes by bottom pillar structure

A three-dimensional model with finite difference and time domain was established to investigate the enhancement of the light output intensity of GaN light-emitting diodes (LEDs) with bottom pillar (BP) structure. Through comparing the normalized light extraction intensity of GaN LEDs with or without BP in different dimensions, the theoretical results show that the light output intensity in the LED with BP structure involved could be enhanced by about 30%. The influence of BP structure on the light output intensity of a LED could be explained by the physical model of light interaction. In addition, the experimental results also show the same trend to the theoretical calculations.

Improved light-output power of GaN LEDs by selective region activation

A novel selective high resistivity region (SHRR) is created under the p-pad metal electrode of a normal GaN light emitting diode. In conventional designs, light generated under the opaque p-pad metal electrode is absorbed or reflected by the contact and lost. In the SHRR design, the area under the p-pad metal electrode is selectively given a higher resistance, reducing current flow and light generation under the contact. Under constant current testing, the current normally passing through the SHRR region is instead distributed over the visible (i.e., useful) area of the device, resulting in significantly increased light-output power and luminous efficiency.

The microstructure investigation of flip-chip laser diode bonding on silicon substrate by using indium-gold solder

In recent year, the combination of III-V semiconductor devices with Si manufacturing techniques to develop optoelectronic integrated circuits (OEICs) has been widely studied. Flip-chip bonding has been used widely because superior electrical performance, proper reliability, efficient heat conduction and self-alignment are the advantages of this technology. Because optoelectronic devices are quite sensitive to temperature and stress-induced degradation, the bonding medium should be chosen to have high thermal conductivity and stress-relief. The indium (In) based alloy solders are generally recognized to provide lower melting point, longer fatigue life, and higher thermal conductivity. In this study, we have successfully developed a fluxless bonding process to manufacture In-Au microjoint between laser diode and silicon substrate. During the soldering, the solder reacts with the bonding pad metal to form the intermetallic compound at the interface. Such an intermetallic compound is crucial to the quality of solder joint. We utilized SEM, EDX, and XRD to observe and identify the intermetallic compounds. These results indicate that AuIn/sub 2/ is the main intermetallic phase and plays an important role on the quality of joints. Moreover the reliability of solder joint is strongly depended on the initial microstructure. The optimum bonding temperature is found to be about 200/spl deg/C by the microstructure of the solder joint by SEM and optoelectronic characteristics (I-V and L-I) of the laser diodes. Shear force test has also been performed according to MIL-STD-883C. The results reveal the fact that all well-boned devices meet the shear force requirement. To verify the thermal stability, the bonded samples were tested by thermal shock test. The bonded specimens endure 500 cycles of thermal shock between liquid nitrogen temperature and a hot plate (80/spl deg/C). To evaluate the long-term reliability, the bonded laser diodes were subjected to an accelerated aging test at 90/spl deg/C for 500 h. These devices show no abrupt degradation from I-V and L-I plots and their mechanical strength is nearly unchanged as before. This shows that indium could achieve the requirements of thermal stability. The flip-chip bonding technique by using indium solder shows good feasibility for the integration of laser diodes on silicon substrates.

Improved Extraction Efficiency of Light-Emitting Diodes by Modifying Surface Roughness With Anodic Aluminum Oxide Film

An anodic alumina oxide (AAO) film with nano-roughening is added on the top window layer of AlGaInP light-emitting diodes (LEDs) to improve the light extraction of the device. The AAO film has a natural porosity to provide light scattering centers at the surface, allowing an increase of light emission intensity with no loss of or damage to the semiconductor material. Further, the fabricated AAO film with a refractive index is about which is intermediate between those of air and the window layer of GaP. By inserting this layer between the ambient and GaP, it broadens the critical angle for light emission and reduces internal reflection. Experiments with laboratory-fabricated AlGaInP devices of conventional design demonstrated a 32% improvement in the luminous intensity at 20 mA for the device with the AAO layer. This letter shows by theory and experiment that AAO films can be used as a low-cost, easily implemented surface nano-roughening for improving extraction efficiency of AlGaInP LEDs.

Titanium nitride as spreading layers for AlGaInP visible LEDs

Thin titanium nitride (TiN) films with low sheet resistance and high transparency were deposited on AlGaInP light-emitting diodes (LEDs) to improve light extraction from the LED surface. Comparison test devices were fabricated both with and without TiN spreading layers. Results show LED current crowding at high current is reduced for devices with TiN current spreading film, improving external efficiency. It is confirmed that TiN films are feasible as current spreading layers of AlGaInP LEDs.

Selective high resistivity region formation by a Ni catalyst on GaN light-emitting diodes

The activation of Mg-doped GaN using a thin Ni film has been investigated. The results show that the Ni film significantly enhances the hydrogen desorption from the GaN film, which could improve the hole concentration and the resistivity of a p-GaN film. By means of a selective high resistivity region (SHRR) with no Ni catalyst involved, the current blocking layer is thus simply developed beneath the p-pad electrode of the GaN-based light-emitting diodes (LEDs). The light-output intensity for the LED chip with a SHRR is significantly increased as compared with the conventional LED chip. This improvement can be explained by both the additional current injection into the effective active layer of the LED by the SHRR structure and the reduction in optical absorption under the p-pad electrode.

TiN Enhancement of Output Performance for Light-Emitting Diodes under High Injection Current

Thin titanium nitride (TiN) films with low sheet resistance and high transparency are deposited on AlGaInP light-emitting diodes (LEDs) to improve light extraction from the LED surface. Comparison test devices are fabricated both with and without TiN spreading layers. At a high injection current, the TiN film spreads the current over a large area of the device and improves the distribution of light emission. Therefore, the current crowding effect is reduced and light-output power is increased. Background theory is discussed and experimental results are presented. It is shown by theory and experiment that the transparent conducting TiN films can be used as inexpensive, easily implemented current-spreading layers for improved global reliability and efficiency of AlGaInP LEDs.

Modifying the improved light-output intensity of AlGaInP-based LEDs by nanoporous alimina

This investigation describes the development of AlGaInP light-emitting diodes (LEDs) with a nanometer diameter of porous anodic alumina (PAA) films which are formed by anodization technique to improve and modify the light extraction efficiency. The pore-widening time was changed for surface modulation to obtain the optimum light extraction efficiency. The diameter of nano-pores varies from 30 nm to 60 nm and the interpore spacing is about 75 nm. The light output intensity of the PAA LEDs with 40 minutes pore-widening is 1.39 times that of conventional LEDs. PAA films can effectively reduce critical angle loss, Fresnel loss and be used as scattering center to improve the light extraction.

The interface microstructure on the reliability of flip-chip laser diode bonding

We have successfully developed a fluxless bonding process to manufacture In-Au microjoint between laser diode and silicon substrate, and studied the interface properties and microstructure of In-Au layer. From XRD, SEM and EDX results indicate that AuIn/sub 2/ is the main intermetallic phase, which plays an important role on the quality of joints. To study the thermal stability, the flip-chip assemblies were then tested by thermal shock and high temperature storage. From the thermal shock test, we can find that high bonding temperature will result in failure mode, such as the occurrence of cracks at the grain boundary. From thermal aging test, no brittle intermetallic phases and fractures were observed in the solder joints. Therefore, the bonding process has to be optimized because the reliability of solder joint strongly depended on the initial microstructure. The optimal bonding temperature is 200/spl deg/C after thermal shock test and high temperature storage test.